A 43 million-year-old transitional form: an amphibious whale

The evolution of whales from a small, deer-like artiodactyl took about ten million years: from about 50 million to about 40 million years ago. That’s remarkably fast evolution, especially when you consider the amount of morphological and physiological change that occurred, and the fact that the divergence between chimps and modern humans from their common ancestor, which (I think) took far less morphological and physiological change, took more than 6 million years.

Fortunately, we have a good fossil record of whales from Egypt, Pakistan and intervening areas, and so we can document this rapid change. Although the closest living relative of whales are hippos, the ancestor of modern cetaceans might have looked something like the first photo below, a reconstruction of the fossil Indohyus, a terrestrial, raccoon-sized artiodactyl (note hooves) similar to the modern chevrotain (“mouse deer”), which is known to stay underwater for long periods to escape predators.

Indohyus (reconstruction)

Here’s a living chevrotain, the Lesser mouse-deer (Tragulus kanchil). It’s hard to imagine that something like this could evolve into the mighty whale!

At any rate, a new paper in Current Biology by Oliver Lambert et al. documents a transitional form between ancient and modern whales, a species they name Peregocetus pacificus, dating from about 43 million years ago. It was found not in the Middle East but in Peru, and so also provides information about how whales colonized the Atlantic Ocean after their presumed origin off Southeast Asia. You can read the paper for free by clicking on the screenshot below, or by getting the pdf here. If somehow your access is blocked, judicious inquiry will get you a pdf.

This “whale” (if it can be called that) was dug up on the southern coast of Peru, with a large proportion of its skeleton remaining (see below; solid lines are recovered bones). From this skeleton we can conjecture that it was amphibious—it could swim and also walk on the land. Here’s the tally of found bones, and a reconstruction of how they were used to both swim and walk. As you can see from the scale, it was about 4 meters long:

(From paper): Schematic drawings of the articulated skeleton of MUSM 3580 showing the main preserved bones, in a hypothetical swimming and terrestrial posture. For paired bones, the best-preserved side was illustrated (sometimes reversed), or both sides were combined (e.g., mandible). Stippled lines indicate reconstructed parts and missing sections of the vertebral column; cranium, cervical vertebrae, and ribs based on Maiacetus inuus.

Some questions:

Why do we think it could swim?Peregocetus had a well-formed pelvis, attached to the rest of the skeleton, and well formed legs that would have stuck well outside the body. It had a long tail, and the configuration of the tail vertebrae, compared to those of other swimming mammals like otters and beavers, suggest that the tail was somewhat paddle-like, powerful, and thus could be used for swimming. (This is seen in the reconstruction at the bottom.). It’s not known if the tail ended with a boneless “fluke” (as in modern whales), as that would not have been preserved.

The rear feet were long, as were the rear digits, and those digits were flattened with flanges on the side, indicative of webbed rear feet. Based on the foot anatomy, the authors suggest that “Propulsive movements were either alternate or simultaneous hind-limb paddling or body and tail undulations, as observed in modern river and sea otters, alternating between lift-based propulsion via pelvic undulations, including tail and hind limbs, and drag-based propulsion via independent strokes of the hind limbs.”

Here’s the distal part of the front leg:

Here’s the rear foot with flattened digits (I believe “3” are the hooflets):

Why do we think it could walk? I’ll quote the authors here: “The fore- and hind-limb proportions roughly similar to geologically older quadrupedal whales from India and Pakistan, the pelvis being firmly attached to the sacrum, an insertion fossa for the round ligament on the femur, and the retention of small hooves with a flat anteroventral tip at fingers and toes indicate that Peregocetus was still capable of standing and even walking on land.”

What else is interesting about this fossil? It had sharp and robust teeth, some shearing teeth, and a long snout, indicating that it probably ate large bony fish. Here is a mandible with teeth:

Further, this fossil suggests to the author a route for migration of ancestral whales from their origin in the Pacific to the Atlantic Oceans and into the New World.

Based on recent finds of whale fossils in West Africa, as well as this find from Peru, the authors suggest that whales made their way through the Tethys sea (the Mediterranean, which was contiguous with the Indian Ocean then), south along the West African coast, then hopped over the South Atlantic (much narrower at that time due to continental drift) along the coast of South America and then south through the Isthmus of Panama (open to the sea then) to get back to the eastern Pacific. It also is thought to have spread north along the east coast of North America. Below you can see the route of migration, with the authors’ hypothesis denoted by the black arrows below. (A trans-Pacific route can’t be ruled out, but the Pacific was much wider then and there are no early whale fossils on the Pacific coasts.)

Finally, here’s a reconstruction of the beast on land and see from the paper’s Supplemental Information:

(From paper): Figure S4. Artistic reconstruction of the middle Eocene (about 42.6 Ma) protocetid whale Peregocetus pacificus gen. et sp. nov., Related to Figure 2 and Data S2. Life reconstruction of two individuals of P. pacificus, one standing on the rocky shore and the other hunting sparid fish, along the coast of nowadays Peru. The presence of a caudal fluke in P. pacificus remains hypothetical and should be tested with the discovery of a more complete specimen, including posteriormost caudal vertebrae. Reconstruction by A. Gennari.

Here we have a true intermediate form, a transitional species which occurs when it’s supposed to: after the earliest whales but before the appearance of modern whales. Given this find, as well as the panoply of other fossil whales showing progression from ancestral to modern forms, creationists will be hard pressed to explain this.

I’m no evolutionary biologist but in consulting a couple of books, Prothero’s “Evolution: What the Fossils Say and Why It Matters” and Carroll’s “Vertebrate Paleontology and Evolution”, it looks like it falls in between Rodhocetus, 48-40 MYA (still with well-developed hind legs) and Takracetus, 45 MYA (quite reduced hind legs and front legs more flipper-like), within the Eocene, though at 43 MYA, there would appear to be some temporal overlap with Takracetus. By 40-35 MYA, Basilosaurus shows up, with little nubbins for hind limbs and the more modern Mysticeti and Odonticeti showing up about 28 MYA in the Oligocene.

At least that’s what I can make out with a quick perusal. Perhaps other, better educated readers can correct me or fill in some gaps?

Well, there is a progressive movement of the ‘nostril’ from the tip of the snout to a more backward position in the Pakistani fossils. This new fossil apparently has no skull, so it is difficult to tell.
I think Creationists are already hard pressed by the Pakistan series, but then, internal convictions are rarely fazed by something as trivial as evidence.

It seems to me the migration over the entire Earth’s oceans would only take a few years or decades, not millions of years. Unless there was a definite land barrier, why, I ask myself, would they not blitz around the world as soon as they became capable of open ocean swimming? That certain forms are found across the world could be accounted for by the incompleteness of the fossil record.

would they not blitz around the world as soon as they became capable of open ocean swimming?

That point is perfectly fair, but I’m not sure that both 4-limb and tail propulsion speaks of “open ocean swimming”. I’m trying to think of a quadruped that gets more than a few tens of miles from land – Ursus maritimus (the maritime bear, otherwise known as the polar bear) and “salt-water crocodiles” (not sure of the species name, or count) are the closest parallels I can think of, and while they can do tens of miles from shore or ice floe they do eventually need to haul out and rest. Having sticky-out limbs would seem to have a penalty.

That doesn’t rule out the trans-Atlantic hop that is hypothesised. When the ocean was narrower, the hop distance is shorter, and there were probably mid-ocean ridge islands, and possibly islands along the W-Africa to N-Brazil transform fault zones. Currently the St Peter & St Paul rocks (Google maps link, try zooming out in satellite view a little, to get the little bubble of aerial photography in a literal ocean of very approximate bathymetry) sit on the submarine junction, and just break surface, while the eastern end of the Azores archipelago provide an example of what can form above transform zones. Playing hopscotch across an ephemeral island chain seems very do-able to me, and an almost architypal situation for speciation in isolated populations. (I’m just trying to scratch my own itch about the present day bathymetry there.)

I didn’t realize there were islands that far out toward the middle of the Atlantic. Yes, I can’t see how it would take more than a mere 1 million years for these creatures to proliferate around the globe. Itch well scratched.

The individual islands are fairly ephemeral – StPeter&StPauls rocks total a bit over a hectare spread between 5 islands, equivalent to a circal just over 120m in diameter. I ended up doing the sums last night, which explained why the 250m resolution bathymetry/ coastline databases I’ve found won’t plot the damned places.
“Megamullion” is a nice word.

More likely Sierra Leone or … I’ll have to look up the runners & riders – the surrounding countries didn’t register with me at all. Guinea or Liberia.
But if the Chinese want it, there’s nothing to stop them buying it. Building an airbase there though would be a challenge, given the steepness of the drop-offs.

This is very interesting. We just had Neil Shubin give the plenary lecture at the Assoc. of Southeastern Biologists Meeting this past week in Memphis, TN, and as you might expect, he talked about transitional fossils. And gave a great talk to boot, especially aimed at young scientists, and students. Very inspirational.

I am somewhat surprised at how rapidly whales evolved, but not how rapidly they increased in size. If anything, it seems changes in size are relatively easy in animals, perhaps because they involve changes in timing of development and not entirely new adaptations. I’d be interested in hearing what others think about this. From a molecular genetics perspective, this seems ripe for investigation.

This transition from small hoofed beast to whale is so fantastic and amazing that it is not surprising that evolution skeptics abound. If it were mine to decide, I would have the evidence behind this transformation be a big part of high school biology and require all students to attend.

Wouldn’t matter anyway. It’s the State Standards that matter, and their relation to the required standardized tests. It’s something like:

“All Hail the Standardized Test! We Bow Down Before Thee and Sacrifice Our Children to Thee in Thy Honor!”

And if that fails to appease the gods, then the heads of teachers, principals, and superintendents are taken and erected on pikes outside the schools to appease the devils and demons known collectively as The Accreditation.

At least I assume it’s something like that; I’m not privy to these secret ceremonies that decide such things.

Are the heads detached from the teachers first, and don’t the local animal rights people have something to say about the treatment of the pike?
I never could understand why people went into teaching. This explains the misgivings.